1. Field
The described aspects relate to support of Voice Call Continuity (VCC) for IP Multimedia Subsystem (IMS) emergency calls (e.g. E911 calls in North America or E112 calls in Europe) made using Voice over IP (VoIP).
2. Background
Voice Call Continuity (VCC) is a feature that is being evaluated by the Third Generation Partnership Project (3GPP) and by the Third Generation Partnership Project 2 (3GPP2). A definition of VCC has been produced by 3GPP in 3GPP TS 23.206 (“Voice Call Continuity (VCC) between Circuit Switched (CS) and IP Multimedia Subsystem (IMS); Stage 2”) and in 3GPP TS 23.237 (“IP Multimedia Subsystem (IMS) Service Continuity; Stage 2”) which are publicly available documents. A definition of VCC has also been produced by 3GPP2 in 3GPP2 X.S0042-A (“Voice Call Continuity between IMS and Circuit Switched Systems”) which is also a publicly available document. Both definitions of VCC are very similar and support continuity of a voice call that was previously established between a wireless terminal and any other device (wireless or non-wireless) when the wireless terminal switches between using a wireless access that supports circuit mode and a wireless access that supports VoIP. In particular, use of VCC reduces the need for releasing a call (e.g. a circuit mode call) and re-establishing a call (e.g. using VoIP) when the user switches access which may cause significant delay and disturbance to the users involved in the call and might result in an inability to re-establish the call.
Examples of wireless access networks that support circuit mode, sometimes referred to as the circuit switched (CS) domain, include the Global System for Mobile Communications (GSM), Universal Mobile Telecommunication System (UMTS) using wideband code division multiple access (W-CDMA) and cdma2000 1X. Examples of wireless access networks that support VoIP include UMTS W-CDMA, cdma2000 1xEV-DO (Evolution Data Optimized) and various wireless LAN (WLAN) networks. In some cases, the same access network (e.g. UMTS W-CDMA) can support both circuit mode and VoIP, although the user's wireless terminal would be required to use just one of them (at any one time) for a particular call.
In the “Voice Call Continuity between CS and IMS” stage 2, 3GPP draft TS 23.206 in Release 7, VCC is mainly supported in a new entity that may be part of the IP Multimedia Subsystem (IMS) in a network and that has been assigned several different names over the course of developing the current solution. In an early version of 3GPP TS 23.206, this entity was known as a VCC call continuity control function (CCCF). In the latest version of TS 23.206 it is called a VCC Application or Domain Transfer Function (DTF). In 3GPP TS 23.237, it is called a Service Centralization and Continuity Application Server (SCC AS). In 3GPP2 X.S0042-A (“Voice Call Continuity between IMS and Circuit Switched Systems—Stage 2”), VCC is supported by an entity similar to these 3GPP entities known as a VCC Application Server (VCC AS). The 3GPP VCC CCCF, 3GPP VCC Application, 3GPP SCC AS and 3GPP2 VCC AS are all defined to be located in the calling wireless user's home network, which is generally incompatible with the current solution for VoIP IMS Emergency calls defined in 3GPP TS 23.167 for Release 7 and Release 8 and with the similar 3GPP2 solution defined in 3GPP2 X.S0049 where IMS VoIP emergency call support is restricted to the visited network. This means that it will generally not be possible to support VCC for IMS Emergency calls unless a new method of VCC is defined for the visited network or unless IMS emergency calls are routed through the home network. Exemplary embodiments of the invention uses the former alternative since home network support of IMS Emergency calls would significantly change the current solution for VoIP emergency calls as well as introduce a number of new problems such as liability issues, support of visited country regulatory requirements and possibly latency issues. Such a home network based solution would also be incompatible with supporting IMS emergency calls from unauthorized users.
Some alternative solutions to support VCC for wireless emergency calls have been defined by 3GPP in TR 23.826. Some problems with these solutions are as follows: (a) it may be difficult for a serving network to know whether a UE originating an emergency call supports VCC and to distinguish between a new emergency call from the UE and a request for domain transfer for an existing emergency call for the UE; (b) it may be difficult, complex and/or expensive to convey any indication obtained by a network MSC related to (a) to any GMLC involved in supporting the call; and (c) the support of VCC for emergency calls by the UE and by an MSC should preferably require little or no new impact.